**Neither a teacher nor a native speaker.**
Your text looks good to me, however, I'm not sure about these four things:
(I added and highlighted them.)
We learn the basic rules of scouting in the first three months
withinby the activities as a mountain bike tour, fishing and hillwalking. At the end of the third months we swear and start scouting activities. We arrange mountain and forest tours and have (an/the) opportunity to learn through having fun.
Within looks strange to me, altough I cannot say it's definitely wrong.
I would add an a before mountain bike tour.
... by the activities as a mountain bike tour, fishing and hillwalking.
"A fishing" or "a hillwalking" seems to be wrong, but the mountain bike tour needs an a (in my opinion).
A confirmation or some advice by other members/teachers would be very nice
Student or Learner